- It is absolutely tiny compared to the other planets.
- It is made of ice, rather like a comet.
- Its "moon" is about the same size, and they actually orbit around each other.
- Its orbit is tilted relative to all other planets in the solar system.
- If Pluto is a planet there are at least three chunks of ice that need to be reclassified as planets, plus almost certainly dozens more in the Kuiper belt, probably hundreds, possibly thousands. (Does it not diminish the value of the term "planet" if there are thousands of tiny balls of ice in all kinds of crazy orbits given that classification?)
- Failing the reclassification of the other ice-balls, it is confusing and scientifically unhelpful having two different classifications for the same type of celestial body.
- And one that I think there is some confusion surrounding: Pluto doesn't exert enough gravitational force to have been able to carve out its own clear orbital path around the sun. The confusion here is in thinking that this is about Pluto's orbit crossing Neptune's. Sure, maybe that's part of it, but the main point, really, is that Pluto's gravity is not strong enough to have diverted (or attracted) a number of comets that travel through its orbital path.
- It's "always" been called a planet.
- People want it to be a planet.
- People don't want to have to explain to their poor easily-traumatised children that Pluto is no longer a planet (heaven forbid they should actually have to explain and discuss the issues with their kids; oh no, kids aren't capable of understanding the science behind it, they'll just be sad about "losing" Pluto).
- Textbooks will have to be reprinted (funny, I seem to remember textbooks being revised almost annually when I was in school).
- We'll have to come up with new mnemonics (and the completely stupid and false corollary that the new mnemonics will somehow be inferior to the old ones).
- It's got the same name as a cute cartoon dog (never mind that Pluto is the most annoying in a painfully annoying cast of characters).
Seriously, people, get over it. Pluto isn't a planet. It never was a planet. It's barely more than a comet.
But, more importantly, this ruling brings Pluto (and its dwarf planet brethren) into the spotlight. Calling Pluto the furthest, smallest planet was doing it a disservice, because that meant that there was not enough acknowledgement of its real differences compared to the other planets (which are, of course, themselves divided into two very different categories). Now the story of Pluto's rise and fall in status will generate discussion of not just the basic concept of planets, but the deeper issues of the many known and unknown objects in the greater solar system. This "demotion" brings with it a sense of wonder: if Pluto is just one object among many, what is yet to be found in the little explored outer reaches of our solar system?